The image tone of a developed silver halide layer describes the perceived color or tint of the nominally black filamentous silver. Depending on a number of factors, the image tone can be described as brown-black or blue-black, and changes in this image tone can be undesirable in certain imaging environments. Blue-black image tone is also variously known as a cold or hard tone, whereas a brown-black tone can be described as a warm, or soft, tone.
The tone of an image can be observed by reflectance or transmission of light, depending on the particular application or format of the imaging medium. In images for medical applications, particularly those produced from exposure by a laser imager, the image is most often viewed by light transmitted through the image, and hence the transmitted image tone is of great importance. The tone and consistency of the tone of a medical image are critical, given the large number of images that need to be viewed, and the limited time available for analysis of each image. The image tone needs to be optimized to provide acceptable image quality, with minimal eye strain on the viewer. To this end a blue-black image tone is highly desirable. At the same time as the tone is optimized, there should be no corresponding increase in the Dmin, or other significant change in sensitometry.
The image tone of a developed photographic emulsion depends on a number of factors, e.g., the silver halide grain size, the degree of gelatin hardening (see i.e., E. Weyde, Photogr Korres., 1, 7 (1962)), and various processing parameters, such as the developer pH, the nature of the developing agent and the drying method (see i.e., T. H. James and W. Vanselow, Photographic Science & Engineering, 1, 104 (1958)). The image tone becomes progressively more yellow-brown with decreasing size of the silver particles within the image areas due to scattering of the blue component of the light. To overcome this undesirable shift in image tone, toning agents have traditionally been added to fine-grained photographic emulsions containing high levels of silver chloride in the grains. However, fine-grained emulsions of high silver bromide content (e.g., 50 to 100 mol % bromide) are difficult to tone.
The perceived image tone may also be modified through a variety of methods including the use of a highly blue-tinted base, the use of a dye-forming developer, or adaptation of the morphology of the developed silver. Of these, the latter is the preferred route, since flexibility in the type of support (clear or blue-tinted) is maintained. Dyes formed during development can be problematical, since they often have low stability. By changing the form of the filamentary silver produced during development, a permanent cold image tone can be constructed.
Several published papers have discussed the tone of developed silver halide emulsions. For example, the toning of silver chloride emulsions with various toning agents is described in K. Futaki, et al., Photographic Science & Engineering, 4, 257 (1960). The growth and nature of silver filaments in silver bromide emulsions is discussed in W. E. Muller, Photographic Science & Engineering, 15, 369 (1971). The effect of the developer on the form of silver filaments is mentioned in T. H. James, et al., Photographic Science & Engineering, 1, 104 (1958). Some factors affecting image tone, such as film hardness is described in E. Weyde, Photogr. Korres., 98, 7 (1962).
U.S. Pat. No. 2,298,093 raises the issue of incorporating toning agents to silver bromide emulsions. Compounds containing the unit --C(.dbd.O)NHC(.dbd.S)Y-- are disclosed as toners of silver bromide emulsions to impart a blue-black image. However, no description was given regarding how the tone was determined nor was any supportive data supplied.
U.S. Pat. No. 4,818,675 and JP 03-153234 have cited the use of blue leuco dyes to mask the warm image tone by imagewise production of a blue dye. The use of blue dyes in the emulsion layer to alter the visual impression of the image color have also been described in EP 0481651 and JP 03-271733. The disadvantage of these approaches are that they can lead to loss of speed and/or increase the optical density of the film in unexposed regions.
Others have disclosed the use of heterocyclic thiols or thiones (see i.e., JP 61-020026 and JP 63-015140). EP 0197895 describes the use of 2-alkylthio-tetrazaindenes as image toners in black and white paper products. Other silver coordinating species discussed in the art include macrocyclic sulfides (see i.e., JP 63-313939 which describes compounds containing at least 3 sulfur atoms, at least one carbon chain and at least one other divalent linking group).
Improvements in image tone have also been discussed in conjunction with a variety of silver grain emulsions. For example, U.S. Pat. Nos. 5,013,641 and 5,258,280 describe improvements of the image tone in tabular emulsions. JP 04-294346 discloses the use of mercaptooxadiazoles in emulsions of particle size 0.4 .mu.m or less to improve the image tone. One of the examples uses a core-shell cubic silver chlorobromide emulsion of mean grain size 0.35 .mu.m in combination with a mercaptooxadiazole which gives a blue-black image tone compared to the yellowish-black tone of the emulsion without the oxadiazole.